A tension type color selecting electrode (hereinafter referred to as a tension mask) such as an aperture grill is used as a color selecting mechanism in a cathode ray tube such as a color television receiver or a color display. The tension mask is prepared by, for example, subjecting a low carbon or ultra low carbon aluminum killed steel to a hot rolling, a cold rolling, a continuous annealing, a secondary cold rolling and, as required, an annealing for removing the residual stress from the steel sheet, followed by perforating the steel sheet by photo etching method, attaching to a frame by loading tension of, for example, 200 to 400 N/mm2 in a single direction or two directions, and applying a blackening treatment to the steel sheet and the frame. The blackening treatment, in which the tension mask is heated to, for example, 450° C. to 500° C. for forming an oxide film of magnetite on the surface, is intended to prevent the rusting and to lower the heat radiation. If the tension of the tension mask is lowered by the creep during the heat treatment, it is possible for various inconveniences to take place. For example, the positions of the holes of the mask are deviated. Also, resonance tends to be caused by the sound from the speaker. Further, it is possible for the electron beams to fail to strike on predetermined positions on a phosphor screen so as to bring about “the color deviation”.
The prior arts intended to improve the creep resistance under high temperatures are disclosed in, for example, JP 62-249339 A, JP 5-311327 A, JP 5-311330 A, JP 5-311331 A, JP 5-311332 A, JP 6-73503 A, JP 8-27541 A, JP 9-296255 A, and JP 11-222628 A. These prior arts teach the idea of suppressing the climbing motion of dislocation by adding Mn, Cr, Mo, etc. as steel components and/or adding a large amount of N as a solid solution element.
In recent years, the television receiver and the computer display have been made larger in size, higher in precision and higher in flatness. In this connection, the deviation in the orbits of the electron beams caused by the external magnetic field such as the magnetic field generated by, for example, the geomagnetism has come to attract attentions as the cause of “the color deviation” in addition to “the color deviation” caused by the creep of the tension mask referred to above. It is of course important to improve the deviation in the orbits of the electron beams noted above for improving the color deviation.
The measures for improving “the color deviation” caused by the deviation in the orbits of the electron beams, i.e., the measures for improving the magnetic shielding properties, are also proposed in various publications. For example, the idea of adding Si to the steel sheet is proposed in JP 63-145744 A, JP 8-269569 A and JP 9-256061 A. The idea of adding Cu to the steel sheet is proposed in JP 10-219396 A. Further, the idea of adding Ni to the steel sheet is proposed in JP 10-219401 A.
However, attentions are not paid to the improvement in the magnetic shielding properties in the techniques proposed in JP 62-249339 A, JP 5-311327 A, JP 5-311330 A, JP 5-311331 A, JP 5-311332 A, JP 6-73503 A, JP 8-27541 A, JP 9-296255 A, and JP 11-222628 A.
On the other hand, the magnetic properties can be certainly improved in the techniques proposed in JP 63-145744 A, JP 8-269569 A, JP 9-256061 A, and JP 10-219396 A. In these techniques, however, the surface defect tends to be generated in the hot rolling process and the recrystallization annealing process of the steel sheet because Si or Cu is added to the steel sheet, making it impossible to apply these techniques to the steel sheet for the tension mask requiring severe surface properties.
Further, the technique proposed in JP 10-219401 A is not desirable because the manufacturing cost is increased by the Ni addition and, in addition, the etching properties of the steel sheet are deteriorated.
As described above, the steel sheet exhibiting excellent magnetic shielding properties with satisfying other properties such as the surface properties and the etching properties have not yet been developed in the prior art. Particularly, it is impossible to obtain nowadays the steel sheet exhibiting both the excellent magnetic shielding properties and the excellent creep resistance under high temperatures.